1. Field of the Invention
The present invention relates to class-D amplifiers. More particularly, the present invention relates to an improvement of class-D amplifiers.
2. Description of the Related Art
Portable electronic devices such as wireless phones or portable music players run on rechargeable batteries. The audio amplifier for driving the speaker or earphone is a major power consuming component in such a portable device. For extended battery lifetime, class-D amplifiers are usually adopted because a class-D amplifier is more efficient than a class-AB amplifier under general circumstances.
FIG. 1 is a schematic diagram showing the circuit of a conventional class-D amplifier 100. Since class-D amplifiers are already well known, it is unnecessary to discuss their operation details here. Briefly speaking, comparator 106 compares the amplified input signal provided by amplifier 104 and the triangle signal provided by ramp generator 118. Pulse width modulation (PWM) logic 108 drives the output stage consisting of metal oxide semiconductor field effect transistors (MOSFETs) Q1-Q4 through gate drivers 110 according to the output signal of comparator 106.
Please refer to FIG. 2. FIG. 2 is a schematic diagram showing voltage waveforms of class-D amplifier 100. Signal 201 is the amplified input signal provided by amplifier 104. Signal 202 is the triangle signal provided by ramp generator 118. Comparator 106 compares signals 201 and 202. Signal 203 is the voltage signal output by MOSFETs Q1-Q4 and received by low-pass filter 116. MOSFETs Q1-Q4 are operated as switches. As shown in FIG. 2, signal 203 is a square wave at a high state when amplified input signal 201 is higher than triangle wave 202 and at a low state when amplified input signal 201 is lower than triangle wave 202. Low-pass filter 116 passes the average value of square wave 203 to loudspeaker 114. Thus loudspeaker 114 is driven by an amplified signal whose waveform approximates the original input signal Vin.
In typical voice communication or music playback, the audio signal generally has frequent pauses for milliseconds or even a fraction of a second. During a wireless phone conversation, typically only one party is in a talking mode, the other party is in a listening mode. The audio amplifier for the talking party is often inactive but still consumes power. Furthermore, when the input signal is below the audible level, a class-D amplifier may actually have a poorer efficiency than that of a class-AB amplifier. This is due to gate charge loss and switching loss. For low power applications such as in cellular phones or portable music players, the gate charge loss and switching loss of a class-D power amplifier are relatively significant.